1. Field of the Invention
The present invention relates to a heat-shrinkable polyester film, and more particularly, to a heat-shrinkable polyester film used for labels or coverings of containers and a method for manufacturing the same, the heat-shrinkable polyester film having superior transparency, superior printing and mechanical properties, superior film-to-flux adhesiveness, optimum shrinking properties and easy of drawing process. 2. Description of the Related Art
Heat-shrinkable films have been extensively used for various purposes, for example, for labeling plastic or glass bottles, batteries, or electrolytic condensers, for covering containers, and for fastening or batch-packing stationary or a plurality of containers. Such heat-shrinkable films are mainly formed of polyvinyl chloride, polystyrene and the like. Recently, use of polyester films as such a heat-shrinkable film is increasing.
Requirements for heat-shrinkable films to be used as wrapping or labeling materials, include a good sealing property and an excellent shrinking property, as well as basic film properties such as heat resistance, chemical resistance, weather resistance and good printability.
Conventional heat-shrinkable films formed of polyvinyl chloride or polystyrene are unsatisfactory in terms of heat resistance, chemical resistance, weather resistance and heat-shrinking properties. In particular, polyvinyl chloride based heat-shrinkable films, which contain chlorine components, have been considered to be unfavorable in terms of an environmental aspect because they produce pollutants during incineration. On the other hand, polyethylene films show inferior printability and thus a special purpose ink, rather than common inks, is needed for application to such films. In addition, its high spontaneous shrinkage ratio make it difficult to be stored for a period of time, and causes failure in a printing process.
Meanwhile, more common heat-shrinkable polyester films formed of polyethylene terephtalate (PET) have a satisfactory heat resistance, chemical resistance, weather stability and shrinking property. However, because shrinkage stress and shrinkage ratio are so high, directly labeling or completely covering a container with a heat-shrinkable polyester film causes various problems. In other words, if the shrinkage ratio of the heat-shrinkable polyester film is too high, nonuniform shrinking results due to the deviation of temperature within a shrinkage tunnel or at the surface of the container. As a result, an image printed on the film is distorted, thereby lowering the commercial usefulness of the product.
Recently, for the purpose of saving storage space, use of rectangular containers is increasing. As shown in FIG. 1, when a conventional polyester heat-shrinkable film is used as a label 11 for such a rectangular container 10, shrinkage stress and shrinkage ratio are high in a direction perpendicular to the major shrinkage direction. Due to a difference in the shrinkage ratio at the edges of the rectangular container 10 and at the flat sides of rectangular container 10, after labeling with a polyester heat-shrinkable film, an edge bowing phenomenon 12 occurs at the edge of the label 11, thereby distorting an image printed on the label 11, giving it a poor appearance.
On the other hand, as for a film to be used as a small label, a shrinkage ratio of 30% or more in 80.degree. C.-water is high enough. As for a film to be used to fully cover a bottle, for example, a beer bottle which must be kept at low temperatures to prevent spoilage of the beer at high temperatures, the shrinkage ratio must be higher even at low temperatures. However, conventional polyester heat-shrinkable films are insufficient for a high shrinkage ratio at low temperatures.
Japanese Laid-open Patent Publication Nos. 63-139725, 7-53416, 7-53737, 7-216107, 7-216109 and 9-254257 taught that the shrinkage rate can be adjusted by blending polyethylene terephtalate or polybutylene terephtalate in a predetermined ratio, or by copolymerizing a dicarbonic component such as a terephtalic acid and an isophtalic acid, and a diol component such as ethylene glycol and 1,4-cyclohexanedimethanol, thereby improving uniformity in shrinkage. Although the suggested methods showed an improvement in terms of uniformity in shrinkage, a shrinkage ratio in the major shrinkage direction is insufficient for use as a full covering for a container. Furthermore, because the shrinkage ratio in a direction perpendicular to the major shrinkage direction is so high, a bowing phenomenon at the edges becomes more serious when it is applied to a rectangular container, thereby deteriorating the appearance of the label with poor transparency.
To improve the edge bowing phenomenon, Japanese Laid-open Patent Publication Nos. 9-239834 and 10-77335 suggested that the shrinkage ratio be adjusted using a polyester copolymer such as neopenthylglycol[2,2-dimethyl(-1,3-propane diol, or by performing a post-drawing process accompanied with a thermal treatment. However, the shrinkage stress in a direction perpendicular to the major shrinkage direction is too high to overcome the edge bowing phenomenon by the suggested method, and the thermal shrinkage ratio in the major shrinkage direction is insufficient to be used as a full covering for a container. As illustrated in FIG. 2, another drawback is that a shrinkable film 21 cannot accurately fit to the shape of a glass bottle 20 near a cap 22, thereby resulting in poor appearance and incomplete sealing. Thus, it is very likely that external contaminants enter the glass bottle 20, causing spoilage of the content.